Close contacts involving germanium and tin in crystal structures: experimental evidence of tetrel bonds

ORIGINAL PAPER

Close contacts involving germanium and tin in crystal structures:

experimental evidence of tetrel bonds

Patrick Scilabra

_{Vijith Kumar}

_{Maurizio Ursini}

_{Giuseppe Resnati}

Received: 9 November 2017 / Accepted: 14 December 2017 / Published online: 8 January 2018 # The Author(s) 2018. This article is an open access publication

Abstract

Modeling indicates the presence of a region of low electronic density (a

Bσ-hole^) on group 14 elements, and this offers an

explanation for the ability of these elements to act as electrophilic sites and to form attractive interactions with nucleophiles.

While many papers have described theoretical investigations of interactions involving carbon and silicon, such investigations of

the heavier group 14 elements are relatively scarce. The purpose of this review is to rectify, to some extent, the current lack of

experimental data on interactions formed by germanium and tin with nucleophiles. A survey of crystal structures in the

Cambridge Structural Database is reported. This survey reveals that close contacts between Ge or Sn and lone-pair-possessing

atoms are quite common, they can be either intra- or intermolecular contacts, and they are usually oriented along the extension of

the covalent bond formed by the tetrel with the most electron-withdrawing substituent. Several examples are discussed in which

germanium and tin atoms bear four carbon residues or in which halogen, oxygen, sulfur, or nitrogen substituents replace one, two,

or three of those carbon residues. These close contacts are assumed to be the result of attractive interactions between the involved

atoms and afford experimental evidence of the ability of germanium and tin to act as electrophilic sites, namely tetrel bond (TB)

donors. This ability can govern the conformations and the packing of organic derivatives in the solid state. TBs can therefore be

considered a promising and robust tool for crystal engineering.

Keywords Tetrel bond . Crystal engineering .

σ-Hole interactions . Supramolecular interactions

Introduction

A comprehensive knowledge of the various interactions (i.e.,

weak bonds) that a molecule can participate in is a

fundamen-tal prerequisite for controlling and designing the conformation

and the packing that the molecule adopts in a crystal.

Interatomic distances that are slightly less than the sum of

the van der Waals radii of the atoms involved (hereafter

termed

Bclose contacts^) are usually (but not always) the

re-sult of attractive interactions between the involved atoms.

Observing the systematic occurrence of close contacts in

crys-talline solids can thus provide great insights into the attractive

interactions that atoms and molecular moieties can participate

in. Close contacts play a crucial role in the properties of

mat-ter, especially condensed phases, and knowledge and control

of these contacts enables the functional properties of

mate-rials—synthetic and natural—to be designed and optimized

[

1

–

3

].

Hydrogen bonds (HBs) are by far the most frequently

oc-curring and widely studied type of interaction [

4

,

5

]; other

weak interactions that have traditionally received attention

include

π–π [

6

], cation–π [

7

], anion–π [

8

], and aurophilic

[

9

] bonds.

σ-Hole interactions [

10

–

12

] represent a relatively

recent entry into the canon of weak bonds [

13

–

15

], but

fol-lowing the seminal papers of P. Politzer et al. [

16

,

17

], these

interactions rapidly became popular targets for studies in this

field [

15

,

18

–

20

]. A covalently bonded atom characteristically

has a region of low electron density, known as the

Bσ-hole,^

which is usually located along the extension of the covalent

bond but on the opposite side of the atom to the bond. The

electrostatic potential in this region is frequently positive and

σ-hole bonding is the result of an attractive interaction

be-tween this positive region (an electrophilic site, the donor site

in the interaction) and a negative site (a nucleophilic site, the

This paper belongs to Topical Collection P. Politzer 80th Birthday Festschrift

* Giuseppe Resnati giuseppe.resnati@polimi.it

1 _{NFMLab—D.C.M.I.C. BGiulio Natta^, Politecnico di Milano, Via L.}

localized nature of the region(s) of positive electrostatic

po-tential. In an R–A···B interaction, where A is the atom with the

positive

σ-hole potential and B is the nucleophile, the angle

R–A···B is generally between 155° and 180°.

Experimental evidence and theoretical calculations

consis-tently show that most of the elements in groups 14–18 of the

periodic table form

σ-hole bonds. A growing consensus is

emerging among chemists that these interactions should be

named according to the group of the periodic table that the

electrophilic atom belongs to [

22

,

23

]. Halogen bonds (XBs)

[

10

,

24

], namely interactions where an atom of a group 17

element is the electrophilic site, represent the best known

sub-set of

σ-hole interactions. Chalcogen bonds (CBs) have been

studied in silico [

25

,

26

] and in the solid [

27

], liquid [

28

], and

gas [

29

] phases. Pnictogen bonds (PBs) have received much

attention in studies performed in silico [

30

] and in the solid

[

31

]; and the aerogen bond (AB) is the most recently

discov-ered subset of

σ-hole interactions [

32

].

The tetrel bond (TB), namely an interaction in which a

group 14 element is the electrophile, has received a great deal

of attention, probably due to the scale of its influence in

chem-istry, e.g., its possible role in SN2 reactions and hydrophobic

interactions [

13

,

33

]. The first convincing evidence of the

ability of carbon to attractively interact with

lone-pair-possessing atoms was reported more than forty years ago. In

1975, Johnson et al. calculated that the arrangement of the

water–carbon dioxide dimer in which there is close C···O

function as electrophiles describe theoretical investigations of

interactions involving carbon [

38

] and silicon [

39

–

41

],

where-as investigations of the heavier group 14 elements are far less

frequent [

42

]. Experimental studies of TBs are quite limited

[

29

,

43

–

45

] and, to the best of our knowledge, they have never

focused on interactions involving germanium or tin. We

there-fore decided to analyze structures in the Cambridge Structural

Database (CSD) in order to assess whether organic derivatives

of these two elements in crystalline solids show the presence

of TBs. We looked for systems in which germanium and tin

form close contacts with nucleophilic sites. Since

directional-ity is a key characteristic of

σ-hole interactions, particular

attention was paid in this survey to the geometrical features

of the observed close contacts, and a linear close contact was

considered to be a TB.

In this paper, we discuss a selected number of crystalline

structures of organic derivatives of germanium and tin in

which these elements form TBs, i.e., close linear contacts with

lone-pair-possessing heteroatoms. Structurally simple and

poorly functionalized molecular systems are preferentially

an-alyzed, as the Ge/Sn···nucleophile interactions that occur in

these systems are more likely to be a straightforward product

of the features of the two sites involved (contributions from

other parts of the molecule(s) are likely to be insignificant).

Wider coverage of organic Ge and Sn derivatives that present

TBs in the solid is given in the works cited in this review. The

interaction distances are analyzed based on the normalized

Fig. 1 Ball and stick representations (Mercury 3.9) of bis(methoxymethyl)phenyl)triphenyltin (MUBVOU, left) and (2,6-bis(ethoxymethyl)phenyl)dichlorophenyltin (LIVHOO, right). TBs are

depicted as black dotted lines; hydrogens have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, green chlorine, red oxygen, dark teal tin

contact (Nc), defined as the ratio between the experimentally

observed separation of the interacting atoms and the sum of

their respective van der Waals radii [

46

].

The use of Nc

values allows linear comparisons between contacts involving

different atoms. While the number of CSD structures in which

Ge/Sn···nucleophile interactions are present is not large

enough to enable definitive and detailed generalizations to

be made, the CSD survey reported here shows that the

forma-tion of attractive interacforma-tions between organic Ge and Sn sites

and a donor of electron density can become a determinant of

structure in crystalline solids. Intra- and intermolecular TBs

are observed, and they can affect the preferred conformation

of a molecule and/or the network of intermolecular

interac-tions in the crystal packing. Importantly, the cases collected

here provide convincing experimental evidence that TBs tend

to be more linear than PBs [

31

].

Oxygen atoms as TB acceptors

The conformation adopted by (2,6-bis(methoxymethyl)

phenyl)triphenyltin (refcode MUBVOU) in the crystal

(Fig.

1

, left) seems to be determined by two intramolecular

Sn···O TBs [

48

]. One interaction distance is slightly shorter

than the other, with the two Nc values being 0.76 and 0.78.

Shorter

σ-hole interactions usually tend to be more linear;

consistent with this characteristic, the two C–Sn···O angles

in the TBs mentioned above are 168.05° and 172.55°,

respec-tively. As discussed above, another common feature of

σ-hole

interactions is that the more electron-withdrawing the residue

covalently bonded to the

σ-hole donor site, the more positive

the

σ-hole, and the closer and stronger the interactions with

incoming nucleophiles. Interestingly, in an analog of the

com-pound discussed above wherein two of the phenyl rings are

replaced with chlorine atoms, the two intramolecular TBs

a r e

m u c h

s h o r t e r ;

i . e . ,

i n

( 2 , 6

-bis(ethoxymethyl)phenyl)dichlorophenyltin (refcode

1_{A van der Waals radius of 210 pm was adopted for germanium, as suggested}

by Batsanov in [47].

Fig. 2 Ball and stick

representations (Mercury 3.9) of

methyl-tris((2-methoxymethyl)phenyl)germane (IMUTEP, top left), bromo-

tris((2-methoxymethyl)phenyl)germane (IMUTAL, top right), chloro-

tris((2-methoxymethyl)phenyl)germane (IMUSUE, bottom left), and

fluoro-tris((2-methoxymethyl)phenyl)germane (IMUSOY, bottom right). TBs are depicted as black dotted lines; hydrogens have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, brown bromine, green chlorine, yellowish green fluorine, red oxygen, light teal germanium

LIVHOO), the Nc values for the Sn···O TBs are 0.66 and 0.78

(Fig.

1

, right) [

49

].

It is extensively documented that the propensity of a

halo-gen atom to form XBs increases with its molecular weight

[

10

], and that the heavier halogens usually form stronger and

shorter XBs than the lighter ones, with both of these behaviors

being independent of the XB acceptor. Similar trends are

ob-served when elements of groups 16 and 15 form CBs and PBs,

respectively. In all cases, this is probably due to the fact that

within a group of the periodic table, the polarizability

in-creases with the molecular weight of the element, and high

polarizability favors an anisotropic distribution of the electron

density around the atom and thus the strength of

σ-hole

inter-actions. It is no surprise [

50

] that

methyl-tris((2-methoxymethyl)phenyl)germane (refcode IMUTEP) shows

only one C–Ge···O contact, and that the corresponding Nc

value (0.87) is greater than the Nc values of the structurally

similar tin derivatives MUBVOU and LIVHOO [

51

] (Fig.

2

,

top left).

Bromine is more electronegative than carbon, and the Br–

Ge···O TB in bromo-tris((2-methoxymethyl)phenyl)germane

(refcode IMUTAL) is shorter (Nc = 0.79) than the C–Ge···O in

IMUTEP (Fig.

2

, top right) [

51

]; chlorine is more

electroneg-ative than bromine, and the Cl–Ge···O TB in

chloro-tris((2-methoxymethyl)phenyl)germane (refcode IMUSUE) is even

shorter (Nc = 0.76) (Fig.

2

, bottom left) than the Br–Ge···O

TB. Also, in these three structures, the linearity of the TB is

correlated with its length (the C–Ge···O, Br–Ge···O, and Cl–

Ge···O angles are 171.79°, 172.64°, and 173.24°,

respective-ly). In fluoro-tris((2-methoxymethyl)phenyl)germane

(refcode IMUSOY), a fluorine is substituted for the methyl

of IMUTEP and the depletion of electron density at

germani-um becomes large enough that two TBs are present (Fig.

2

,

bottom right). Consistent with the relative electronegativities

of fluorine and carbon, the F–Ge···O interaction is closer and

more directional than the C

–Ge···O interaction (the Nc values

for the Ge···O separations are 0.78 and 0.95, respectively).

Also, the presence of a tin-bonded iodine atom can promote

the formation of close contacts (Fig.

3

). Two independent

molecules are present in the unit cell of crystalline

iodo(2,6-bis(methoxymethyl)phenyl)diphenyltin (refcode RAKBOV),

Fig. 3 Ball and stick representation (Mercury 3.9) of the two molecules of the unit cell of iodo(2,6-bis(methoxymethyl)phenyl)diphenyltin (RAKBOV). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, purple iodine, dark teal tin

Fig. 4 Ball and stick representations (Mercury 3.9) of one of the two independent molecules in the unit cells of (Z)-2-methyl-4-phenyl-3-(trimethylgermanyl)but-2-enoic acid (QIBDOV, left), (2-carbomethoxy-1,4-cyclohexadien-1-yl)trimethyltin (KASYOS, middle), and trans-N-t-butyloxycarbonyl-2-methyl-6-(trimethylstannyl)-4-phenyl)piperidine

(EABFES, right) derivatives. TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, light blue nitrogen, light teal germanium, dark teal tin Fig. 5 Ball and stick representations (Mercury 3.9) of 1D chains generated by ethyl trimethyltin diazoacetate (SIWRAR, top) and 2,5-bis(trimethylgermyl)thiophene-1,1-dioxide (QAHXIG, bottom). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, light blue nitrogen, yellow sulfur, light teal germanium, dark teal tin

and in both of them the conformation is locked in by two

intramolecular TBs: an I–Sn···O and a C–Sn···O TB. The

dis-tances of the former interactions are shorter and those

interac-tions are more directional than the latter ones (the Nc values

are 0.70 and 0.72 for I–Sn···O and 0.79 and 0.81 for C–Sn···O;

the mean I

–Sn···O angle is 166.19° and the mean C–Sn···O

angle is 166.68°).

Carbonyl oxygen atoms can act as effective TB acceptors.

In (Z)-2-methyl-4-phenyl-3-(trimethylgermanyl)but-2-enoic

acid (refcode QIBDOV) [

52

], a short C–Ge···O contact is

present in both conformations adopted by the compound in

the crystals (Fig.

4

, left) (Nc for Ge···O is 0.80; the C

–Ge···O

angles are 174.17° and 175.00°), and a shorter TB occurs in a

trimethylstannylcarbomethoxy derivative (refcode KASYOS)

[

53

], where a similar tin-based tecton is present (Nc for C

–

Sn···O is 0.76) (Fig.

4

, middle). Similar TBs are given by the

carbonyl oxygens of carbamates (e.g.,

N-t-butyloxycarbonyl-2-methyl-6-trimethylstannyl-4-phenyl-piperidine, refcode

Fig. 6 Ball and stick representations (Mercury 3.9) of the 1D networks formed by N-triethylstannylsuccinimide (FUSZIC) due to N–Sn···O TBs (top), by N-chlorosuccinimide (CSUCIM) due to N–Cl···O XBs (middle), and by N-bromosuccinimide (NBSUCA) due to N–Br···O XBs (bottom). T h e t h r e e m e t h y l g r o u p s o f t h e e t h y l r e s i d u e s o f N -triethylstannylsuccinimide and hydrogen atoms have been deleted for the sake of simplicity. TBs and XBs are depicted as black dotted lines and green dotted lines, respectively. Color code: gray carbon, red oxygen, purple iodine, brown bromine, dark teal tin

Fig. 7 Ball and stick representation (Mercury 3.9) of the 1D network in which the ketone oxygen of O-tricyclohexyltin-4-oxo-4-phenylbutanoate (APAZIB) functions as the TB acceptor site. Hydrogen atoms and five of the cyclohexyl carbons have been deleted for the sake of simplicity. Color code: gray carbon, red oxygen, dark teal tin

Fig. 8 Ball and stick representation (Mercury 3.9) of the two-dimensional network formed by bis(tricyclohexyltin)nonanoate (CUXSOF). Five atoms of the cyclohexyl residues bound to tin have been deleted for the sake of simplicity. Color code: gray carbon, red oxygen, dark teal tin

EABFES; Nc = 0.75 and the C–Sn···O angle is 165.31°; Fig.

4

, right) [

54

,

55

] and several other carbonyl derivatives, e.g.,

amides [

56

], aldehydes [

57

,

58

], and ketones [

59

].

The CSD contains both intra- and intermolecular TBs that

have a carbonyl oxygen acting as the TB acceptor and

facili-tate the generation of discrete adducts [

60

] or infinite chains

(one-dimensional networks, 1D nets). In ethyl trimethyltin

diazoacetate (refcode SIWRAR) [

61

], the diazoacetate residue is

expected to form a

hole on tin that is more positive than the

σ-holes formed by the methyl groups. Consistent with this expectation,

a tetrel-bonded infinite chain is present in the crystal of the

com-pound (Fig.

5

, top), wherein the carbonyl oxygen approaches the tin

atom along the extension of the N2C–Sn covalent bond (the Sn···O

separation is 312.5 pm, which corresponds to an Nc value of 0.85;

the C–Sn···O angle is 176.46°). Similarly, the most positive σ-hole

on germanium in 2,5-bis(trimethylgermyl)thiophene-1,1-dioxide

(refcode QAHXIG) [

62

] is expected to occur opposite to the

O2

SC–Ge covalent bond, and an infinite chain (Fig.

5

, bottom) is

formed in which the sulfonyl oxygens approach germanium atoms

along the extension of each O2

SC–Ge covalent bond, leading to a

particularly linear geometry (the Ge···O separation corresponds to an

Nc value of 0.97, and the C

–Ge···O angle is 179.77°).

N-triethylstannylsuccinimide (refcode FUSZIC) [

63

] is a

self-complementary module that forms tetrel-bonded infinite

chains (one-dimensional networks, 1D nets) (Fig.

6

, top).

Consistent with the expected involvement of an sp

lone pair

of the carbonyl oxygen as the nucleophilic site that interacts

with Sn along the extension of the N–Sn covalent bond, the

Sn···O=C angle is 138.28° and the tin atom is approximately

in the plane of the succinimide (the distance between the mean

square plane through the seven heavy atoms of the

succinimide moiety and the tetrel-bonded tin atom is

219 pm). The halogen-bonded infinite chains formed by

N-chloro- and N-bromosuccinimide (refcodes CSUCIM01 and

NBSUCA, respectively) [

64

] are also reported in Fig.

6

(mid-dle and bottom) in order to highlight the analogous

supramo-lecular features of TB and XB.

In several structures in the CSD, the tin atom of a

trialkylalkanoyltin moiety found in R3Sn–OC(O)R′

deriva-tives shows the presence of a TB with a carbonyl oxygen

located opposite to the covalent Sn–O bond, and

one-dimensional [

65

], two-dimensional [

66

], or

three-dimensional [

67

] networks are formed depending on the

over-all structure of the compound (Figs.

7

,

8

, and

9

).

Various other oxygen functionalities can act as donors of

electron density to organotin and germanium derivatives, e.g.,

water [

68

–

70

], sulfoxides and sulfones [

71

–

74

], as well as

Fig. 9 Ball and stick representation (Mercury 3.9) of the three-dimensional network with adamantanoid topology formed by bis(tri-n-butyltin)-1,2,2-trimethylcyclopentane-1,3-dicarboxylate (DIYFIB). Three atoms of the butyl residues bound to tin and the methyl pendants on the cyclopentyl rings have been deleted for the sake of simplicity. Color code: gray carbon, red oxygen, dark teal tin

Fig. 10 Ball and stick representations (Mercury 3.9) of the trimer formed by 1,3-bis(bromodimethylstannyl)propane and water (XINROB, top left), of the dimer formed by bro mo-tris( p-ethylphenyl)tin and hexamethylphosphoramide (HEVQIJ, top right), of the dimer formed by chlorotrimethyltin and triphenylphosphine oxide (HIGRUK01, bottom left), and of the dimer formed by chlorotriphenyltin and dimethyl sulfoxide (RUGYOI, bottom right). Hydrogen atoms and the 2,2′-bipyridine in XINROB have been deleted for the sake of simplicity. TBs are depicted as black dotted lines. Color code: gray carbon, red oxygen, blue nitrogen, orange phosphorus, green chlorine, brown bromine, yellow sulfur, dark teal tin

phosphine oxides, hexamethylphosphortriamide, and their

an-alogs [

75

–

81

] (Fig.

10

).

Nitrogen atoms as TB acceptors

The CSD contains several structures in which the nitrogen

atom of amine, pyridine, and cyano moieties forms a close

contact with a tin or germanium atom (Fig.

11

), thus showing

that—similar to oxygen atoms—nitrogen atoms can act as TB

acceptors, and this can be the case whether there is sp

, sp

, or

sp hybridization.

The ability of nitrogen atoms of tertiary amines to form

close contacts with organogermanium and -tin derivatives is

particularly well documented. For instance, two

symmetrical-ly nonequivalent molecules are present in crystals of

tris(2-((dimethylamino)methyl)phenyl)germane (refcode

GAGYIW) [

82

], and the conformations of both molecules

are influenced by three intramolecular C–Ge···N TBs

(Fig.

12

, left) (the Nc values of these TBs span the range

0.82–0.84, and the C–Ge···N angles vary between 172.45°

and 176.79°). The C–N–C angles vary between 109.70° and

113.25°, indicating that nitrogen atoms of the tertiary amine

moieties adopt a tetrahedral conformation and the lone pairs

align with the extensions of the covalent C

–Ge bonds, as

expected for

σ-hole interactions (the C–N···Ge angles span

the range 82.34–120.39°).

Imine nitrogen atoms behave in a similar manner to amine

nitro-gens. A close linear C–Ge···N interaction affects the conformation

adopted by

1-(trimethylsilylimino(diphenyl)phosphoranyl)-2-(triphenylgermyl)benzene (Nc for Ge···N is 0.85; the C

–Ge···N

angle is 173.79°) (refcode VIQXIC) [

83

] (Fig.

12

, right). In the

crystal of this compound, the P=N···Ge angle is 96.80°, and the

germanium atom is approximately in the iminophosphoranyl plane

(the distance between the tetrel-bonded germanium atom and the

mean square plane through the phosphorus, nitrogen, and silicon

atoms is 263 pm), suggesting that the lone pair at nitrogen aligns

with the extension of the covalent C–Ge bond.

Close intramolecular Ge···N contacts affect the conformation of a

family of 4,6,11-trioxa-1-aza-5-germabicyclo[3.3.3]undecanes

(germatrane derivatives). In the solid, 5-(t-butyl)-germatrane

(refcode BUWBUQ) [

84

] adopts an endo conformation (Fig.

13

,

left) where the C

–Ge···N separation is as short as 223.6 pm (Nc =

0.61). 5-Bromogermatrane (refcode BUWCUR) [

85

] behaves

sim-ilarly (Fig.

13

, middle), and the Br–Ge···N separation is even shorter

Fig. 11 Ball and stick representations (Mercury 3.9) of the dimer formed by chloro(trimethyl)tin and pyridine (CMEPSN, top left), of the trimer formed by chloro(tribenzyl)tin and 4,4′-bipyridyl (FEJFUW, top right), and of the 1D chain formed by chlorotris(4-cyanobenzyl)tin (BIBQIN, bottom). Hydrogen atoms have been deleted for the sake of simplicity. TBs are depicted as black dotted lines. Color code: gray carbon, blue nitrogen, green chlorine, dark teal tin

Fig. 12 Ball and stick representations (Mercury 3.9) of tris(2-((dimethylamino)methyl)phenyl)germane (GAGYIW, left) and 1( t r i m e t h y l s i l y l i m i n o 1( d i p h e n y l ) p h o s p h o r a n y l ) 2 -(triphenylgermyl)benzene (VIQXIC, right) derivatives. TBs are depicted

as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, yellow sulfur, pearl white silicon, orange phosphorus, light teal germanium

(208.4 pm, Nc = 0.57) than in BUWBUQ, consistent with the fact

that bromine is more electronegative than carbon and the

σ-hole

opposite the Br–Ge covalent bond is probably more positive than

that opposite the C–Ge bond. Analogous endo conformations and

Ge···N distances that are much shorter than the sum of the van der

Waals radii of the germanium and nitrogen atoms are observed in

other germatrane derivatives [

86

–

88

] and related systems [

89

,

90

]

(Fig.

13

, right). Similar behavior is encountered in the crystals of tin

analogs. 5-Methyl-1-aza-5-stannabicyclo[3.3.3]undecane (refcode

FEWXOU) [

79

] and its 5-fluoro [

91

], 5-chloro [

92

], 5-bromo

[

91

], and 5-iodo [

91

] analogs (refcodes ZANKEE, DAYMUL,

ZANKOO, ZANKUU, respectively) all show close Sn···N contacts

(Fig.

14

).

As in organogermanium derivatives, the nitrogen atom of the

2-(dimethylaminomethyl)phenylstannyl moiety forms an

intramo-lecular TB which affects the conformation of the respective

com-pound in the solid. This is the case for

(cyclopenta-2,4-dien-1-yl)-(2-(dimethylaminomethyl)phenyl)diphenyl tin (refcode

IHOZAH) [

93

] (Fig.

15

, left), where the intramolecular C–Sn···N

distance corresponds to an Nc value of 0.74 and the C–Sn···N angle

is 171.08°, congruent with an attractive interaction between the lone

pair of the tertiary amine nitrogen and the

σ-hole along the extension

of the C

–Sn covalent bond. Analogous Sn···N interactions are

pres-ent in structurally related derivatives [

94

–

96

]. A five-membered and

tetrel-bonded ring similar to that of IHOZAH is afforded by

(3-aminopropyl)triphenyltin (refcode COKVUV) [

97

] (Fig.

15

, right),

which shows an Sn···N interaction where Nc is 0.74 and the C–Sn···

N angle is 175.81°.

The tin atom of R3Sn

–OC(O)R′ derivatives is a good TB

donor and frequently interacts with the oxygen atom of a

carbonyl group (Figs.

7

–

9

) or the nitrogen atom of a pyridine

moiety. The intermolecular Sn···N interaction occurs opposite

to the covalent Sn–O bond, and discrete trimers [

98

] (Fig.

16

,

top) or one-dimensional [

99

–

102

] (Fig.

16

, bottom) or

two-dimensional [

103

] networks (Fig.

17

) are formed depending

on the ability of the tin derivative to function as a mono-, bi-,

or polydentate tecton.

The nitrogen atom of pyridine derivatives forms close

con-tacts with tin along the extensions of not only O–Sn bonds but

also C–Sn, Cl–Sn, Br–Sn, I–Sn, and S–Sn bonds [

99

,

101

,

104

,

105

]. In all cases, the geometric features of the adducts

indicate that the nitrogen lone pair is oriented along the

Fig. 13 Ball and stick representations (Mercury 3.9) of 5-(t-butyl)germatrane (BUWBUQ, left), 5-bromogermatrane (BUWCUR, middle), and phenyl(tris(2-(trimethylsilylamido)ethyl)amine-N,N′,N″)germanium (XUSLOM, right). TBs are depicted as black dotted

lines; hydrogen atoms and methyl substituents on the silyl moieties of XUSLOM have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, light blue nitrogen, bronze bromine, pearl white silicon, light teal germanium

Fig. 14 Ball and stick representations (Mercury 3.9) of 5-methyl-1-aza-5-stannabicyclo(3.3.3)undecane (FEWXOU, left) and 5-fluoro-1-aza-5-stannatricyclo(3.3.3)undecane (ZANKEE, right). TBs are depicted as

black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, yellowish green fluorine, dark teal tin

extension of one of the covalent bonds of tin. For instance, in

the infinite chain formed by the dithiocarbamate reported in

Fig.

18

(refcode UGEFIX), the S

–Sn···N angle is 174.50°, the

geometry around the nitrogen is strictly trigonal planar, and tin

is nearly in the pyridine plane (the two C(sp

)N···Sn angles are

121.18° and 122.40°, and the distance of tin from the mean

square plane through the pyridine ring is 85 pm).

The cyano group seems to be able to act as an effective TB

acceptor group via the lone pair at the nitrogen. Moreover, due

to its strong electron-withdrawing ability, it is expected that

when the cyano group is directly bound to a tin or germanium

atom, the

σ-hole opposite the covalent NC–Sn/Ge bond will

be particularly positive. Indeed, trimethyltin cyanide (refcode

TIMSNC01) and dimethyltin dicyanide (refcode DMCYSN)

are both self-complementary modules that form infinite chains

[

106

] and square 2D networks [

107

], respectively (Fig.

19

), by

pairing TB donor and TB acceptor sites. Dimethylgermanium

dicyanide (refcode DMCYGE) shows somewhat similar

behavior.

Tetrakis(2-cyanobenzyl)tin (refcode JIWROX) [

108

]

(Fig.

20

) functions as a self-complementary tecton, as the

cyano group of one molecule aligns with the extension of

one of the C

–Sn covalent bonds of an adjacent molecule,

Fig. 15 Ball and stick representations (Mercury 3.9) of (cyclopenta-2,4-dien-1-yl)-(2-(dimethylaminomethyl)phenyl)diphenyltin (left) and (3-aminopropyl)triphenyltin (IHOZAH, right). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, dark teal tin

Fig. 16 Ball and stick

representations (Mercury 3.9) of the trimer formed by (ferrocene-1-carboxylato)triphenyltin and 4,4′-bipyridine (IVUVUR, top) and of the infinite chain formed by

(pyridine-4-carboxylato)tricyclohexyltin (UZAVUN, bottom). TBs are depicted as black dotted lines; hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, orange iron, light blue nitrogen, dark teal tin

Fig. 17 Ball and stick representation (Mercury 3.9) of the network g e n e r a t e d b y d i ( t r i n b u t y l ) s t a n n y l 5 ( ( p y r i d i n 4 -ylmethylene)amino)isophthalate with 4,4′-bipyridine (TISVEY). TBs are depicted as black dotted lines; three atoms of the butyl residues at tin and hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, red oxygen, light blue nitrogen, dark teal tin

ultimately forming infinitely long tetrel-bonded ribbons

(Nc = 0.96; the C–Sn···N angle is 178.46°).

In 2-(dimethylaminomethyl)phenyl)cyanodiphenyltin and

bis(2-(dimethylaminomethyl)phenyl)dicyanotin (refcodes

WUVKOP and WUVLOQ, respectively) [

109

], one and two

NC–Sn···N close contacts are present, respectively, and the

amine nitrogen acts as the TB acceptor site in all cases

(Fig.

21

). This may suggest that a N(sp

) atom is a better TB

acceptor than a N(sp) atom. The same ability to act as a donor

of electron density is observed in XB formation.

Halogen atoms as tetrel bond acceptors

Structures in the CSD reveal that the four halogens F, Cl, Br,

and I can all form close contacts with tetravalent germanium

and tin atoms in organic derivatives. These interactions can be

rationalized as TBs due to the fact that the halogen atom is

located approximately along the extension of one of the

cova-lent bonds formed by the germanium or tin. The bond with the

most electron-withdrawing group is preferentially involved in

the formation of these close contacts.

For instance, crystals of bis(2,5-bis(trifluoromethyl)

phenyl)(dichloro)germane (refcode ZAVCUW) have two

symmetrically nonequivalent molecules in the unit cell

[

110

]. Both of these molecules show two fairly short and

linear TBs oriented along the extensions of the Cl–Ge bonds

(Nc values span the range 0.78–0.79; the C–Ge···F angles are

between 176.15° and 174.93°) (Fig.

22

, left). Analogously, an

values are shown close to the respective interactions. Color code: gray carbon, yellow sulfur, light blue nitrogen, dark teal tin

Fig. 19 Ball and stick representations (Mercury 3.9) of the 1D infinite chain formed by trimethyltin cyanide (TIMSNC01, top) and the 2D network generated by dimethyltin dicyanide (DMCYSN, bottom). TBs are depicted as black dotted lines; hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, dark teal tin

black dotted lines; hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, dark teal tin

Fig. 21 Ball and stick representations (Mercury 3.9) of the conformations adopted by cyano-2-(dimethylaminomethyl)phenyl)diphenyltin (WUVKOP, left) and bis(2-(dimethylaminomethyl)phenyl)dicyanotin (WUVLOQ, right). TBs are depicted as black dotted lines; hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, light blue nitrogen, dark teal tin

intramolecular C

–Ge···F close contact locks in the

conforma-tion of (1,2,3,3,3-pentafluoroprop-1-en-1-yl)triphenyl

ger-manium (refcode ADUKUH) [

111

] in the solid and allows for

the formation of a tetrel-bonded five-membered ring (the Nc

value of Ge···F is 0.86; the C–Ge···F angle is 166.80°). The tin

analog of ADUKUH (refcode ADUKOB) behaves similarly,

as an intramolecular C

–Sn···F TB is present in both of the

independent molecules present in the unit cell of the crystal,

leading to a tetrel-bonded ring (Fig.

22

, right).

Fig. 22 Ball and stick representations (Mercury 3.9) of the conformations adopted by bis(2,5-bis(trifluoromethyl)phenyl)(dichloro)germane (ZAVCUW, left) and (1,2,3,3,3-pentafluoroprop-1-en-1-yl)triphenyltin (ADUKOB, right). TBs are depicted as black dotted lines; hydrogen atoms have been deleted for the sake of simplicity. Nc values are shown close to the respective interactions. Color code: gray carbon, yellowish green fluorine, light teal germanium, dark teal tin

Fig. 23 Ball and stick

representations (Mercury 3.9) of 1D chains generated by fluorotricyclohexyltin (BAJWOY, top),

dichlorodimethyltin (DMSNCL, middle), and dibromodiethyltin (DESNBR, bottom). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, brown bromine, green chlorine, yellowish green fluorine, dark teal tin

Fig. 24 Ball and stick representations (Mercury 3.9) of the conformation adopted by tetrakis(2-chlorobenzyl)tin (CEWGEQ01, left) and the network generated by tetrakis(chloromethyl)tin (UGATEB, right). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, green chlorine, dark teal tin

CEWGEQ) [

117

] provide a nice example of intramolecular

C–Sn···Cl interactions, as three such contacts (Nc values range

from 0.94 to 0.97) lock in the molecular conformation

(Fig.

24

, left). Interestingly, tetrakis(2-methoxybenzyl)tin

(refcode HEVFOD) [

118

] and tetrakis(2-fluorobenzyl)tin

(refcode VULSOM) [

119

] present four intramolecular C–

mine atom is localized along the extension of the Cl–Sn bond

(Nc = 0.78; the Cl–Sn···Br angle is 172.06°) (Fig.

25

, bottom)

[

121

]. Similar Cl–Sn···Br contacts are present in various other

(6-bromo-1,2-dihydroacenaphthylen-5-yl)tin derivatives.

Bromine atoms can also be involved in intermolecular TBs.

Fig. 26 Ball and stick representations (Mercury 3.9) of the conformation adopted by bromo(4-iodo-1,2,3,4-tetraphenyl-1,3-butadienyl)diphenyltin ( S I C S O M , b o t t o m ) a n d t h e n e t w o r k f o r m e d b y tris(trimethylstannyl)ammonium iodide (RONDAZ, top). One layer of RONDAZ is presented, and hydrogen atoms have been omitted for clarity. TBs are depicted as black dotted lines; Nc values are shown close to the respective interactions. Color code: gray carbon, purple iodine, brown bromine, light blue nitrogen, dark teal tin

Fig. 25 Ball and stick representations (Mercury 3.9) of the conformation adopted by chloro(6-bromo-1,2-dihydroacenaphthylen-5-yl)diphenyltin ( V E K K U T, b o t t o m ) a n d t h e 1 D c h a i n g e n e r a t e d b y 3 β-(bromodimethylstannyl)-24-nor-5β-cholane (MISYAO, top). TBs are depicted as black dotted lines; hydrogen atoms have been omitted for clarity. Nc values are shown close to the respective interactions. Color code: gray carbon, brown bromine, dark teal tin

T h i s i s t h e c a s e i n t h e s t e r o i d d e r i v a t i v e

3β-(bromodimethylstannyl)-24-nor-5β-cholane (refcode

MISYAO) [

122

] (Fig.

25

, top), crystals of which include

infi-nitely long 1D chains assembled via Br–Sn···Br.

The covalent bond pathway connecting iodine and tin in

(8-iodo-1-naphthyl)trimethyltin (refcode AQIVUS) [

123

] is

reminiscent of that connecting bromine and tin in VEKKUT,

and this translates into a supramolecular similarity between

the C–Sn···I TB in the former compound and the Cl–Sn···Br

TB in the latter. In the crystal structure of

bromo(4-iodo-1,2,3,4-tetraphenyl-1,3-butadienyl)diphenyltin (refcode

SICSOM) (Fig.

26

, bottom) [

124

], the iodine atom acts as

the TB acceptor and approaches tin—the TB donor—along

the extension of the Br–Sn bond (Nc = 0.94; the Br–Sn···I

angle is 168.95°). This pattern is consistent with the fact that

the most positive

σ-hole on tin is expected to occur at this

position, as bromine is more electron-withdrawing than the

other atoms bound to tin. Finally, the Sn···I interactions

pres-ent in crystals of tris(trimethylstannyl)ammonium iodide

(refcode RONDAZ) [

125

] provide a nice example of

charge-assisted TB. The existence of this type of TB further

high-lights the similarities of the different subsets of

σ-hole

inter-actions, as charge-assisted XBs [

126

] and charge-assisted PBs

[

31

] have already been observed. Specifically, two

crystallo-graphically independent salt units are present in the crystal of

R O N D A Z ; i n b o t h o f t h e s e u n i t s , t h e

tris(trimethylstannyl)ammonium cations act as tridentate TB

donors and the iodide anion as a tridentate TB acceptor, and

3D networks are formed (one 3D network is shown in Fig.

26

,

top).

Conclusions

In this paper, we have reported the results of an analysis of the

CSD that aimed to identify crystal structures of organic

deriv-atives of germanium and tin in which these two elements form

close contacts with lone-pair-possessing atoms.

We focused our attention on close contacts where oxygen,

nitrogen, and halogens were the lone-pair-possessing atoms,

as a wide range of examples of those close contacts were

found in the CSD. However, it may be worth mentioning that

other heteroatoms (e.g., sulfur [

127

–

129

] and phosphorus

[

130

–

132

]) also form similar interactions. Ether and carbonyl

oxygens as well as amine, pyridine, and cyano nitrogens can

all be involved in such interactions, and the geometries

ob-served indicate that the lone pair of the heteroatom is directed

towards the germanium/tin atom independent of the

hybridi-zation of the oxygen/nitrogen atom (which can be sp

, sp

, or

sp). Close contacts are formed by derivatives in which

germa-nium and tin atoms bear four carbon residues or where there

are halogen, oxygen, sulfur, or nitrogen substituents instead of

one, two, or three of those carbon residues. Regardless of the

nature and hybridization state of the lone-pair-possessing

at-om, and independent of the nature of the residues that are

covalently bound to germanium and tin, the close contacts

are preferentially formed along the extensions of the covalent

bonds that germanium and tin form with strongly

withdrawing residues. Moreover, the more

electron-withdrawing the residue bound to germanium/tin, the closer

the interaction along the extension of that bond.

All of these features are typical of

σ-hole interactions, so

we propose that the close contacts described in this review

should be termed tetrel bonds. Tetravalent germanium and

tin atoms have a tetrahedral geometry. When these atoms form

one or two close contacts with lone-pair-possessing atoms, the

surrounding geometry tends to change to a trigonal

bipyrami-dal or octahedral geometry, respectively. These changes can

be explained as being due to sp

→ dsp

or sp

→ d

sp

rehybridization at the tetrels. They can also be rationalized

by invoking the presence of a tetrel bond [

133

]—an attractive

interaction between a lone pair and a positive

σ-hole along the

extension of a covalent bond formed by the tetrel. The

pres-ence of

σ-holes on all four tetrels is widely supported by

modeling [

38

–

42

], and is also in accord with the

experimen-tally determined geometric features of the interactions

discussed in this review. The rationale for tetrel bonding is

congruent with the other alternative explanations mentioned

above. However, it may offer the additional advantage that

these interactions of group 14 elements can be considered to

be analogous to similar interactions that occur when groups

15–18 elements function as electrophilic sites.

The examined dataset is too limited to be able to draw

general conclusions, but it seems to suggest that the

devi-ation of a tetrel bond from the extension of the relevant

covalent bond to a germanium or tin atom is usually

small-er than the corresponding deviations for most PBs and CBs

[

31

]. This is consistent with theoretical calculations which

show that the region of most positive electrostatic potential

opposite to a covalent bond deviates from the extension of

the bond to the greatest extent in pnictogen derivatives and

to the least extent in tetrel derivatives [

134

,

135

]. The

greater linearity of TBs may be related to the fact that

the electronic asymmetry generated around germanium and

tin atoms by the four residues bonded to them is usually

smaller than the electronic asymmetry generated around

pnictogen and chalcogen atoms by the residues bonded to

them and their lone pair(s) [

136

].

It also seems that steric congestion around the tetrel atoms

studied in this paper plays an influential role in tetrel bond

formation; such steric congestion may even prevent tetrel

bond formation. For instance, tetrakis(2-fluorobenzyl)tin

(refcode VULSOM) forms four intramolecular TBs, whereas

its tetrakis(2-chlorobenzyl) analog (refcode CEWGEQ) forms

t h r e e i n t r a m o l e c u l a r T B s ; a l s o , m e t h y l t r i s ( ( 2

-methoxymethyl)phenyl)germane (refcode IMUTEP) forms

a molecule and/or the network of intermolecular interactions

in the crystal lattice. Tetrel bonds appear to be sufficiently

reliable that they could prove useful tools in crystal

engineering.

Acknowledgements The authors are pleased to recognize the seminal role of Prof. Dr. Peter Politzer in understanding the interactions discussed in this paper as well as sister interactions, all of which are now grouped together under the termBσ-hole bonds.^ The authors are also grateful to Prof. Politzer for fruitful discussions and collaborations regarding this topic.

Open Access This article is distributed under the terms of the Creative C o m m o n s A t t r i b u t i o n 4 . 0 I n t e r n a t i o n a l L i c e n s e ( h t t p : / / creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appro-priate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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